With the decrease in the reserves of conventional crude oils, there is increasing use in petroleum refineries of heavy hydrocarbons such as those extracted from oil sands. These heavy hydrocarbons typically are geographically located in regions remote from refineries that can process them. Consequently, the hydrocarbons need to be transported to a refinery, most usually through a pipeline.
Presently the most convenient method for pipelining heavy hydrocarbons is by mixing the hydrocarbon with a diluent such as natural gas condensate to lower the viscosity and density of the hydrocarbon to render it suitable for pipelining. Experience has shown, however, that in order to meet the pipeline viscosity specifications, more diluent is used than is necessary to meet the density specifications. Moreover, there is growing concern that the supply of natural gas condensate may not keep pace with the continuing growth in use of such heavy hydrocarbons.
An alternate approach to modifying the viscosity and density of heavy hydrocarbons to a range suitable for pipelining involves subjecting the hydrocarbon to "partial upgrading" by hydroconversion, especially slurry hydroprocessing. By "partial upgrading" is meant to subject the hydrocarbon to hydroprocessing under conditions and for a time sufficient to reduce the viscosity and density to pipeline specifications. Partial upgrading, however, reduces the bitumen viscosity to a greater extent than the API gravity so that at the target density the viscosity of the product is significantly lower than that which is required. Additionally, depending upon the level of conversion, among other factors, phase separation of asphaltenes may occur.
Thus, there remains a need for modifying the viscosity and density of heavy hydrocarbons which does not result in asphaltene phase separation.
Additionally, there remains a need for an improved method for rendering heavy hydrocarbons pipelineable while using reduced amounts of diluent.